Unexpected error when assiging COM object to a C# 'dynamic' variable

Question

I am interested in the possibilities offered by the C# dynamic keyword, but I am facing unexpected issues.

If I create a default .NET 4.8 template in Visual Studio 2022...

This works:

dynamic obj = Marshal.GetActiveObject("Excel.Application");
obj.Workbooks.Add();

This works too:

object obj1 = Marshal.GetActiveObject("FancySoft.Application");
object obj2 = obj1.GetType().InvokeMember("FancyProperty", BindingFlags.GetProperty, null, obj1, null);

This fails with a System.Runtime.InteropServices.COMException: 'Error HRESULT E_FAIL has been returned from a call to a COM component.':

dynamic obj1 = Marshal.GetActiveObject("FancySoft.Application");
object obj2 = obj1.FancyProperty;

Finally, if I add a COM reference to FancySoft to my project, this works as expected:

var obj1 = (Application) Marshal.GetActiveObject("FancySoft.Application");
var obj2 = obj1.FancyProperty;

I have found an interesting discussion thread that suggests that a Vendor can intentionally restrict the use of the dynamic keyword in client applications.

In my case, the Vendor doesn't even officially support .NET-based automation scenario (although it is a common scenario in our industry) so I can't ask them for that type of information.

Have you seen this behavior elsewhere? Am I missing something? Is there any way to confirm that this behavior is intentional without asking the vendor? (By looking at the .TLB files, using some reflection tricks, etc.?)

Here is a comprehensive doc to handle your issues. learn.microsoft.com/en-us/dotnet/framework/interop/… — Hamlet Hakobyan
– Hamlet Hakobyan, Commented Nov 16, 2023 at 17:26
dynamic mostly (it not entirely) uses IDispatch ("aka late binding" which is name-based), and yes this is completely possible that it doesn't work. I doubt it's intentional, more a bad interface design, as demonstrated in other post stackoverflow.com/questions/77478817/… — Simon Mourier
– Simon Mourier, Commented Nov 16, 2023 at 17:50
@SimonMourier I wrote a minimal vbscript tonight, which works as expected with the Name property from that other post. That should too be using IDispatch right? If so why a different behavior? — JBL
– JBL, Commented Nov 16, 2023 at 22:55
Well vbscript internals are not the same as .net internals, there are many ways to use IDispatch. Difficult to say more w/o a reproducing COM object to test this specific IDispatch by myself — Simon Mourier
– Simon Mourier, Commented Nov 17, 2023 at 0:04
Thanks @SimonMourier, I learn a lot with every answer. Unfortunately it is highly unlikely that you have access to this program. I just tried the same example in a VB.NET project, and the property can be accessed as one would expect (regardless whether obj1 is defined as an Object or an Application). I would have thought the behavior in VB would be closer to C#, but that's apparently not the case. — JBL
– JBL, Commented Nov 17, 2023 at 11:17

JBL · Accepted Answer · 2025-03-11 16:27:15Z

0

The vendor fixed (at least in part) their COM implementation in a recent release: the dynamic keyword now works as expected. So does the dynamic view when debugging the COM objects.

answered Mar 11 at 16:27

JBL

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